The term which has occasionally been employed by German organ-builders
to designate an open labial stop, the pipes of which were cylindrical,
of small scale, and properly made of tin in the best examples. The
stop, voiced on wind of low pressure, yielded a tone of an extremely
refined and delicate character, probably closely resembling that of
the Vox Angelica, 8 ft., of today.
It was usually and properly made of unison (8 ft.) pitch but an Octave
seems to have sometimes been made.

Adlung lists Vox Virginea with the following description:

Vox virginea, the “maiden's voice” (Jungferstimme),
is perhaps identical to the Jungfernregal
mentioned above in §.161. Its voicing must be gentler (lieblicher)
than that of the abovementioned Vox humana.
It is likely of narrower scale, an octave higher,
and also usable only in the upper octaves.